package day2_linkedlist.leetcode;

/**
 * @Author: yuisama
 * @Date: 2021/9/1 13:45
 * @Description:合并链表
 * https://leetcode-cn.com/problems/merge-two-sorted-lists/description/
 * 将两个升序链表合并为一个新的 升序 链表并返回。新链表是通过拼接给定的两个链表的所有节点组成的
 * 输入：l1 = [1,2,4], l2 = [1,3,4]
 * 输出：[1,1,2,3,4,4]
 */


public class Num21 {
    // 虚拟头结点法
//    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
//        ListNode dummyHead = new ListNode(-1);
//        ListNode temp = dummyHead;
//        while (l1 != null && l2 != null) {
//            if (l1.val <= l2.val) {
//                temp.next = l1;
//                l1 = l1.next;
//            }else {
//                temp.next = l2;
//                l2 = l2.next;
//            }
//            temp = temp.next;
//        }
//        temp.next = (l1 == null) ? l2: l1;
//        return dummyHead.next;
//    }

    // 递归
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        // 终止条件
        if (l1 == null) {
            return l2;
        } else if (l2 == null) {
            return l1;
        } else if (l1.val <= l2.val) {
            l1.next = mergeTwoLists(l1.next, l2);
            return l1;
        } else {
            l2.next = mergeTwoLists(l1, l2.next);
            return l2;
        }
    }
}
